1. Field of the Invention
This invention relates generally to apparatus for recording video and audio signals, and more particularly is directed to such an apparatus suitable for recording an audio signal with its time base compressed.
2. Description of the Prior Art
Helical scan type video tape recorders (VTRs) are widely used in which a magnetic tape is helically wrapped around a tape guide drum with a wrap angle slightly larger than 180.degree.. In such widely used or standard VTRs, two rotary magnetic heads, located at an angular spacing of 180.degree. from each other on the tape guide drum, are rotated at a frame frequency, that is, 30 revolutions per second, and the rotary magnetic heads alternately contact the magnetic tape while the latter is transported at a predetermined or standard tape speed so that the rotary heads are operative alternately to record a video signal in successive slant or skewed tracks on the tape. In such standard VTRs, a video signal for one field is recorded in each skewed or slant track on the magnetic tape, and the audio signal is recorded in an audio track extending along one longitudinal edge portion of the tape. In a playback or reproducing mode, the successive skewed or slant tracks are scanned alternately by two similar rotary magnetic heads for reproducing the video signal from the magnetic tape, and a stationary head scans the longitudinally extending audio track for reproducing the audio signal therefrom.
From the standpoint of increasing the utility of the VTR, it is desirable that the latter be made small in size and light in weight. Further, when the VTR is small-sized and light in weight, it becomes possible to provide a unit in which such VTR is conveniently combined with a television camera. An important factor in limiting the reduction in size of the VTR is the tape guide drum and the associated mechanism for guiding the tape therearound. Therefore, a prerequisite to substantially decreasing the overall size of the VTR has been effecting reduction of the diameter of the tape guide drum therein.
However, if the diameter of the tape guide drum is merely reduced, the recording pattern on the tape guided by the small drum will not coincide with the recording pattern on a tape recorded in a standard VTR. Further, when a television camera and VTR are combined in a portable unit, it may be desirable to use such VTR only for recording, and to employ an existing or standard VTR for playback of the recorded tape. Therefore, it is desirable that the recording pattern effected with the reduced-diameter drum be the same as that achieved with the standard VTR.
In order to provide the same recording pattern as that provided by the conventional or standard VTR, it is enough that the magnetic tape be wrapped around the reduced-diameter tape guide drum with an increased tape wrap angle, provided that each track on the magnetic tape is scanned by a head located at substantially the same position on the drum. However, when high density recording of the video signal is to be effected, adjacent video tracks must be provided in a guard band-less format by respective rotary heads having gaps with different azimuths, that is, extending in different directions. Accordingly, the recording must be effected by two rotary heads with different gap azimuths, and the two rotary heads, of necessity, cannot occupy the same location on the drum, that is, the two heads must be at a distance from each other on the drum.
The above described problems have been solved in a manner disclosed in detail in U.S. Pat. No. 4,510,538 which has a common assignee herewith, and in which there is provided a video signal recording apparatus with a small-diameter tape guide drum which can achieve the same recording format as that provided by the conventional or standard VTR having two rotary magnetic heads with an angular spacing of 180.degree. therebetween. In the video recording apparatus of that patent, the tape wrap angle about the small-diameter tape guide drum is increased, and suitable time base compression of the video signal supplied to the recording apparatus from an associated television camera is realized by changing the speed with which the horizontal lines are scanned in the television camera. However, the recording of the audio signal remains unchanged, that is, the tape transport speed is the same as that in the standard VTR and the audio signal is recorded, without special processing, in the usual audio track extending along a longitudinal edge portion of the tape. By reason of the relatively low tape transport speed, the audio signal recorded in the longitudinally extending audio track by a fixed head is of relatively poor quality or fidelity.
As VTRs with so-called Hi-Fi audio achieved by an AFM (audio frequency modulation) system have become available, a demand has developed for the provision of the combined VTR and television camera with similar high-quality audio recording. In the AFM system, for example, as disclosed in U.S. Pat. No. 4,490,751, also having a common assignee herewith, a carrier is frequency-modulated with the audio signal and is frequency-multiplexed with the usual frequency modulated luminance component and the frequency-converted chrominance component, whereupon the frequency-multiplexed audio, luminance and chrominance are recorded by the rotary heads in the skewed tracks on the magnetic tape. The last identified patent provides the AFM system in an otherwise standard VTR, that is, a VTR with a relatively large diameter tape guide drum having two rotary heads spaced 180.degree. from each other. However, when the AFM system is applied to a VTR having a reduced-diameter tape guide drum with an increased tape wrap angle and with suitable time base compression of the video signal for maintaining the same recording format as the standard VTR, a problem arises. More particularly, since the AFM system records the audio signal by the same rotary head or heads used for recording the video signal, it becomes necessary to process or compress the time base of the audio signal in a manner compatible with the time base compression of the video signal.